1. Field of the Invention
The present invention relates to an organic EL drive circuit and an organic EL display device using the same organic EL drive circuit. In particular, the present invention relates to an organic EL drive circuit IC capable of reducing luminance unevenness on a display screen in the low tone region between drive circuit IC's driven by the current tone system, and an organic EL display device using the same organic EL drive circuit IC.
2. Description of the Related Art
A drive circuit for driving passive matrix type organic EL elements and resetting the organic EL elements by grounding anodes and cathodes of the organic EL elements is disclosed in JPH9-232074A.
On the other hand, a drive circuit of a liquid crystal display device, which drives a data line by a D/A converter circuit, is known. When a plurality of such drive circuits of the liquid crystal display device are applied to pixel circuits of an active matrix type organic EL display panel and are housed in the display panel, down sizing of the organic EL display panel is difficult as disclosed in JP2000-276108A.
However, when the organic EL drive circuit for driving the active matrix type EL display panel is provided externally of the display panel, the down sizing of the organic EL display panel can be achieved to some extent. In such case, write of drive current is performed by charging each of capacitors of pixel circuits, whose capacitance is usually several hundreds pF, with drive current in the order of 0.1 μA to 10 μA. However, when luminance of the active matrix type organic EL display panel is to be tonally controlled, highly precise drive current having minimum current value of about 1 nA to 30 nA is required. There are two types of flowing direction of the drive current, the sink type and the source type. Voltage of a power source line +Vcc is presently about 10V to 20V regardless of the type of the organic EL display panel, the passive matrix type or the active matrix type.
Incidentally, luminance unevenness in the low tone region of a display screen tends to become conspicuous due to visual sensitivity of human. Drive current in the low tone region becomes small and, when a capacitor or an organic EL element, which is a capacitive load, is driven, a portion of the driving current, which does not attribute to light emission, is consumed by initially charging the load. Therefore, it is impossible to obtain sufficient light emission corresponding to a drive current difference, so that luminance unevenness tends to occur. In view of this fact a voltage drive system is proposed for the low tonal region, in lieu of the current drive system.
However, when a voltage drive circuit is used as the drive circuit of the current tone system, there is a problem that the circuit size of the drive circuit is increased correspondingly. In addition, in the current sink type display panel, the voltage for resetting a capacitor of a pixel circuit of an organic EL display panel becomes in the vicinity of the voltage of the power source line +Vcc. Therefore, variation of reset voltage between drive circuit IC's tends to occur, resulting in conspicuous difference in luminance in a boarder area between the drive circuit IC's in the low tone region.